The present invention relates to an apparatus for forming a toner image on a recording medium by using an electrophotographic system, such as a copying machine, a printer, or facsimile equipment. In such an apparatus, toner is used for image development. To be precise, toner is consumed during development. This invention, in particular, relates to a method for detecting and determining a state that exists shortly before the toner runs out. In other words, this invention particularly relates to a method of assessing when there is a low toner state. Furthermore, this invention relates also to an assessment of when there is a no toner state (i.e., a state in which the toner is substantially used up). Also, this invention relates to a method of assessing when an image development apparatus is in an abnormal state with respect to toner. Collectively, the low toner state, the no toner state, and the abnormal state may be referred to as toner states or as toner conditions of an image forming apparatus.
An image forming apparatus is comprised of an exposure controlling means for generating a light beam corresponding to an image forming signal, a photoconductive member for forming an electrostatic latent image upon receiving the light beam, a developing means for forming a toner image by causing the electrostatic latent image to attract a toner via a development roller, and a transfer means for transferring the toner image onto a recording medium.
In such an image forming apparatus, because printing is effected by causing the toner to be attached to the recording medium, the toner is consumed by printing, so that it is necessary to manage the remaining amount of toner.
For this reason, generally, the amount of toner in a developing device is optically detected by providing the developing device with an optical window for detection, or the amount of toner is detected by providing a switching means which is operated on the basis of the quantity of the toner. With the former method, however, there is a problem in that the optical window becomes stained by the toner with the lapse of the time, making it impossible to detect the amount of toner accurately. Further, the latter method involves a problem, among others, that the arrangement of the mechanism becomes complex.
To overcome such problems, a technique has been proposed in which, as disclosed in the specification of the U.S. Pat. No. 5,204,699,the toner in a developing device is estimated by calculating and summing the amount of toner consumed for each pixel on the basis of image forming signals for forming an electrostatic latent image. Further, a technique has been proposed in which, as disclosed in Japanese Patent Application Laid-Open No. 22067/1984, a toner image for testing is formed by a developing device by effecting exposure in a predetermined shape on a photoconductive member, and the amount of toner in the developing device is estimated on the basis of the optical density of the toner image.
However, with the technique in which the amount of toner consumed is estimated by using the image forming signals as in the former case, a situation can occur in which a difference arises between an estimated value and an actual amount of consumption even with respect to an identical image signal owing to a change in the amount of toner attached due to a change in the environment and the like because of a change over time of the environmental conditions and an electrophotographic process element, and although it is determined in accordance with the estimated value that there still remains a sufficient amount of toner, the toner has been actually depleted, and a misprint is outputted.
In addition, with the latter technique in which the toner image for testing is used, the following problem is encountered. That is, although the amount of toner can be detected accurately in a state in which the toner on the surface of a development roller for constituting the developing device has been newly supplied, immediately after the suspension of a developing operation, a sufficient amount of toner is attached to the development roller irrespective of the remaining amount of toner, so that the toner image for testing is formed with a high density, possibly causing an error in the determination of the amount of toner.